Bone marrow aspiration is to take a small number of bone marrow cells for smear examination. Through the cytological examination of bone marrow smear, we can understand the production of various types of cells in the bone marrow, the morphology of various types of cells, the change of their composition, as well as the discovery of abnormal cells, etc., in order to clarify the diagnosis, to observe the efficacy of the treatment, and to evaluate the prognosis. In the course of treatment of hematological diseases, repeated bone marrow aspiration is often needed to analyze the therapeutic effect and guide the next step of treatment. Bone marrow is the most important blood-forming organ, and many blood diseases originate from bone marrow. The hematopoietic cells in the bone marrow will also change after a blood disease, so taking a small amount of bone marrow and analyzing the hematopoietic cells in it, combined with the clinic, can often make a definite judgment of some blood diseases and support a certain degree of diagnosis in the clinic. Diseases that can be diagnosed by bone marrow aspiration include various types of leukemia, various types of multiple myeloma, Gosheimer’s disease, Niemann’s disease, metastasis of cancers in other parts of the body, and certain primary infectious diseases, etc. It is also possible to diagnose some complex diseases, such as iron-deficiency anemia, aplastic anemia, and thrombocytopenic purpura.